Configuration assign and unassign methods should be in Configuration models.py

[Doug Shelley]

Currently the methods for configuration group assign and unassign are located in instances/models.py. It has been suggested that these methods would be more appropriately located in the configuration model.

<esmute> in reality, i would have prefer that the unassign_configuration() and assign_configuration() were in the configuration model and not the instance model (similar to assigning security groups). That way this specific validation method that only pertains to assigning configurations, lives in configurations